University officials said the suspected shooter was in custody, and police spokesman JT Fowler identified him as 18-year-old Steven Jones, believed to be a freshman at the campus.

The shooting happened in a car park outside Mountainview Hall dormitory at the north east side of the Flagstaff campus.

The university called the shooting an "isolated and unprecedented" incident and said classes will go on as planned.

On its website, the university said the campus had been stabilised and the campus was not on lockdown.

Student Maria Gonzalez said that at first she suspected firecrackers.

"I was studying for an exam so I looked out the window and see two people running, and that's when I realised they weren't fireworks they were actually gunshots," she said.

"How am I supposed to feel safe where I'm learning?"

School public relations director Cindy Brown said the injured were taken to Flagstaff Medical Centre, which said it could not release any information on conditions.

The university and Flagstaff police are investigating.

The four-year-old public university has more than 25,000 undergraduates, and plays a major role in the northern Arizona city of Flagstaff

The shooting came on the day Barack Obama was visiting Roseburg, Oregon, where eight students and a teacher were shot and killed last week at Umpqua Community College.

The gunman in the Oregon shooting wounded nine others before turning the gun on himself.

A fraternity based in Ohio said some of its members were involved in the shooting.

Iowa City-based Delta Chi Fraternity said its members were involved, but executive director Justin Sherman said in the statement that the organisation did not have detailed information about the victims.

Mr Sherman said it was an isolated incident and not fraternity-related.

Mr Fowler said the attacker used a handgun in the shootings.

School president Rita Cheng said: "This is not going to be a normal day at NAU. Our hearts are heavy."

She called it an "isolated and unprecedented incident" and confirmed classes would go on as scheduled.

The student who died was identified by the school as Colin Brough.

The victims being treated at Flagstaff Medical Centre are Nicholas Prato, Kyle Zientek and Nicholas Piring.

Alex McIntosh, a friend of Mr Zientek, said he worked part-time at High Country Conference Centre while attending the school full-time.

"He's very calm, very respectful, has a great manner, calm demeanour and you'd never expect him to be caught up in something like this," Mr McIntosh said.

Freshman Cameron Sands, 18, said he had pledged at a fraternity and was due to move into Mountainview Hall.

"It's crazy. You don't think this stuff happens. When I think of Flagstaff, I think safety," he said.